Method for seismic marine survey

ABSTRACT

A method for seismic prospecting in a body of water which comprises trailing a gun adapted to initiate explosively-operated charges at a sufficient depth below the water&#39;s surface to allow the creation in the body of water of high-pressure waves, and detecting the pressure waves with a detector assembly including a housing having a flexible, sound-transmitting wall defining a chamber, an incompressible liquid completely filling the chamber, and a detector probe completely immersed in the incompressible liquid.

REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 354,269 filed Apr. 25, 1973now U.S. Pat. No. 4,059,819, issued Nov. 22, 1977 and acontinuation-in-part of my copending application Ser. No. 115,360, Feb.16, 1976, and now abandoned. An underwater gun which initiates theexplosions detected by the method of this invention is described in mycopending application Ser. No. 354,269, now U.S. Pat. No. 4,059,819,assigned to the same assignee.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,592,286 describes a marine seismic survey system, knownas the MAXIPULSE SYSTEM®, which is based on the recognition that auseful seismic marine survey can be obtained with small explosives bycorrecting for or beneficially employing "bubble" pressure pulses. Thisis accomplished by obtaining a representation of the unreflectedpressure waves in the body of water, created by the explosions of thesmall explosive charges, simultaneously with the conventional detectionof the reflected acoustic waves, and suitably using both of the detectedwaves.

Upon detonation of each charge, the chemical energy in the charge issuddenly converted into kinetic energy of a rapidly expanding masscontained in a bubble. Because the gas bubble is normally generated at adepth of say 30 to 50 feet, it cannot vent directly to the atmosphere.This gas bubble therefore undergoes a very fast initial expansion whichcauses the surrounding water to become suddenly strongly compressed.Subsequent to the initial expansion, the bubble contracts then againexpands then again contracts, etc. The entire sequence of such gasbubble expansions and contractions forms a high-pressure wave in thebody of water which produces reflected, seismic, very-low pressureacoustic waves.

The marine seismic survey described in said U.S. Pat. No. 3,592,286 ispredicated upon the ability to faithfully and substantially-linearlydetect the unreflected, high-pressure wave created by each explosion.

Conventional detectors generally employed in the seismic industry cannotwithstand such high-pressure waves which are like shock waves. The knownseismic detectors comprise a fragile detector which converts acousticenergy into electric energy. More specifically, the detector produces ahigh-impedance charge or voltage when subjected to pressure. Thus, thedetector acts as a pressure-to-voltage converter.

Since in a MAXIPULSE SYSTEM the detector need be positioned a distanceof about 200 to 300 feet from the recording equipment, any resistancevariations in the long line coupling the detector to the recordingequipment may cause significant variations in the current impressed onthe line by the high-impedance voltage from the detector. Accordingly,conventional seismic detectors would fail under the impact of thegenerated shock waves in the body of water, and their usefulness wouldbe greatly reduced by the fact that such detectors act ashigh-impedance, voltage generators.

In sum, the detectors or hydrophones normally used in the seismicindustry are required to detect very-low pressure acoustic waves,whereas the instantaneous shock wave resulting from an explosion mayexceed 10,000 psi. There is therefore a need for a linear shock wavedetector adapted for use in a MAXIPULSE SYSTEM in order to faithfullyreproduce the high-pressure wave resulting from explosions of smallseismic explosive charges.

A high-pressure detector probe is manufactured and sold by the PCBPiezotronics Corporation of Buffalo, N.Y. under several models, althoughthe model of particular interest herein is Model 113A22. The detectorprobe is contained in an elongated metal casing housing a quartz crystaldetector coupled to an amplifier which must be continuously energized.The detector probe is designed to be installed in the wall of a vesselthe inside volume of which undergoes high-pressure fluidic variations.The end of the cylindrical probe which contains the input terminals tothe probe is not exposed to and is protected from the shock waves, andonly the other end of the cylindrical probe containing the quartzcrystal is exposed to and communicates with the inside of the vessel.

By mounting the proble so that only the crystal side faces or is indirect fluid communication with the sea water, it was found that theprobe's metal housing, which constitutes one terminal for the two-wireinput, picks up electric noise signals from the sea water which are onthe order of magnitude of the desired signals to be detected.

After considerable experimentation, I have found that by fully immersingthe probe inside a dielectric fluid housed in a container having aflexible wall, not only does the probe not become damaged (as wasgenerally believed that it would) but the metal housing of the probebecomes electrically isolated from the electric noise currents normallyexisting in the sea water and resulting from man-made objects and fromnatural phenomena. The container with the probe inside thereof will behereinafter referred to as the detector assembly.

The location of the detector assembly on the gun relative to the chargelauncher portion is governed by the ability of the probe to withstandthe large shock waves generated in the ambient water by the explosionsfollowing the detonations of the charges and the ability of the probe toreproduce faithfully the pressure waveforms resulting from suchexplosions.

It is preferred to employ two such detector assemblies displaced fromeach other, so that in the event of a premature detonation which wouldtake place too close to the first detector assembly, thereby affectingits output linearity, the output from the second detector assembly wouldprovide a faithful reproduction of the pressure shock wave.

SUMMARY OF THE INVENTION

A seismic method and apparatus for generating shock waves resulting fromthe explosion of small seismic charges in a body of water by employing adetector assembly which faithfully and accurately converts the impingingshock waves into corresponding trains of current pulses. The detectorassembly has a housing made of a flexible material and defining achamber filled with an incompressible, dielectric liquid. Completelyimmersed in the dielectric liquid is a detector probe which includes aquartz crystal and an amplifier for converting the high-impedancecharges or voltages produced by the crystal, in response to theunreflected shock waves, into corresponding low-impedance currents. Theprobe is energized through a pair of long wires leading from a seismicvessel which tows the gun that creates the explosions. The generatedcurrents by the probe are transmitted to recording equipment on boardthe vessel via the same pair of wires that energize the probe. Therecorded current pulses are subsequently used in the processing of thedetected reflected seismic waves resulting from the detonation of thecharges in the body of water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view partly in section of the detector assemblyof this invention;

FIG. 2 is a view on line 2--2 in FIG. 1;

FIG. 3 is a schematic representation of a seismic survey systememploying an underwater gun for detonating small seismic charges and onwhich is mounted the detector assembly of this invention;

FIG. 4A is an internal schematic representation of the probe detector;and

FIG. 4B is a circuit diagram of the probe detector.

The detector assembly of this invention is generally designated as 10.It includes a housing 12 having a wall 14 made from a flexible materialand preferably having an elongated cylindrical configuration, as shown.The inside volume of cylindrical housing 12 defines a chamber 16 whichis confined between the inner surface of wall 14 and two end caps 18 and20. Wall 14 is clamped tightly to end caps 18 and 20 by suitable clamps22 and 24, respectively. Cap 20 preferably has a conical configurationand includes a fill plug 26 through which an incompressible, dielectricliquid 28 can be poured into and out of chamber 16. A suitable suchliquid is kerosene known in the seismic art as "cable fluid."

Mounted inside chamber 16 and completely immersed in the liquid 28 is adetector probe, generally designated as 30, which is commerciallyavailable from said PCB Piezotronics, Inc., especially Model No. 113A22.The probe has an elongated, cylindrical, stainless steel housing 31(FIG. 4A) which grounds one of the pair of wires 32, 33 connected to theinput terminals of the probe. Housing 31 may have a length of 1.2 inchesand a diameter slightly less than a 1/4 inch.

As shown in FIGS. 4A and 4B, inside housing 31 and near its end wall 34is a quartz crystal 36 mounted to be responsive to the applied pressurein the direction of the arrow. Inside housing 31 are also: an inputresistor 38 and a ranging capacitor 40, both connected across the inputterminals of an integrated-circuit (I-C) amplifier 42 whose outputcircuit is connected to wires 32, 33.

Quartz crystals, because of their extreme ruggedness and high-frequencyresponse, are well suited for the measurement of shock waves. Theemployed quartz crystal 36 preferably operates in a thicknesscompression mode and generates an electric charge whenever deflected bythe applied pressure. This charge creates a voltage across capacitor 40which is fed to the gate of amplifier 42. While this voltage has a highimpedance, the output circuit of amplifier 42 has a very-low impedance(less than 100 ohms) so that essentially the output circuit of theamplifier appears as a current source rather than as a voltage source.

The amplifier operates best when energized by a substantially-constantcurrent source 43 on the deck of a seismic vessel 44. Source 43 isconnected to the remote ends of the pair of wires 32, 33 (FIG. 3), whichpreferably form a coaxial cable 46. The detected pressure shock waveapplied onto the quartz crystal 36 becomes faithfully converted by theprobe 30 into a corresponding current wave at the output of amplifier42, which appears on cable 46.

Cable 46, which applies current to amplifier 42 as well as transmits thedetected current wave from the amplifier, passes through end cap 18. Theremote end of cable 46 is A-C coupled to a recording device 49 on thedeck of the seismic vessel.

The separation between the detector assembly 10 and the recording devicenormally is on the order of 250 feet. Accordingly, if the pressuretransducer assembly 10 of this invention was not acting as a currentsource, variations in the resistance of the long cable 46 might createcorresponding variations in the detected electric signals, therebyintroducing substantial errors and precluding the detector probe fromfaithfully reproducing the incoming shock waves.

Housing 31 of the detector probe 30 is mounted inside and extends onboth sides of a center bore 51 in an annular bushing 50 (FIG. 1) whichis positioned approximately midway between end caps 18 and 20. Bushing50 is secured to wall 14 by an external clamp 54. Between the oppositeflat walls of bushing 50 extend a plurality of longitudinal bores 56which allow fluid communication between, and hence equal pressure in,the two half sections of chamber 16 separated by the bushing 50.

The transducer assembly 10 is detachably coupled by tape bands 56 to aknown seismic gun 58, such as is used in the MAXIPULSE SYSTEM, having anacceleration barrel 60 and a firing head 62. Gun 58 is adapted topercussion initiate a small seismic cartridge 64 and to eject it intothe surrounding body of water 66. Each explosively-operated cartridge 64is delivered to gun 62 by a stream of water flowing through a flexibleconduit 68 and is percussion initiated by a percussion member 70 andejected through a lateral open window 72.

Cartridge 64 includes a delay fuse to allow the seismic boat to tow awaygun 58 by a predetermined distance, typically 6 feet, from the initiatedand ejected charge 64.

When at a safe distance the charge explodes and forms a rapidlyexpanding volume of gas confined to a bubble 74. Because this gas bubbleis normally generated at a depth of say 30 to 50 feet, it cannot ventdirectly to the atmosphere and, therefore, undergoes a very fast initialexpansion, as indicated by the outwardly-directed arrows, which causesthe surrounding water to suddenly become strongly compressed, therebycreating a shock wave. Subsequent to the initial expansion, the bubblecontracts to 75, as indicated by the inwardly-directed arrows, thenagain expands to 76, then again contracts to 77, etc.

The entire sequence of such gas bubble expansions and contractions formsa high-pressure wave in the body of water. It is the object of detectorassembly 10 to faithfully convert the unreflected, high-pressure waveinto a corresponding current wave from which the amplitude of eachpressure pulse and its time of occurrence can be determined.

The high-pressure wave will of course propagate through the water, reachthe sea floor, traverse through the underlying earth formations, becomereflected therefrom and return back as low-energy, reflected, acousticpressure waves into the body of water. The reflected seismic acousticwaves are detected in a conventional manner by a streamer cable 80 alsotowed by the seismic boat.

As previously mentioned, said U.S. Pat. No. 3,592,286 describes a methodfor practicing the MAXIPULSE SYSTEM by correlating the detectedreflected seismic waves by cable 80 with the unreflected pressure wavesdetected by detector assembly 10 of this invention.

Should an undesired premature explosion take place, the previouslymentioned safe distance of about 6 feet may be reduced to one foot andconsequently the resulting shock wave may overdrive the first detectorprobe, thereby resulting in an unfaithful reproduction of the shockwave. There is therefore provided a second detector assembly 10'separated from the first detector assembly 10 by a distance of say 5 to7 feet. The second detector assembly 10' is in all respects identical tothe first detector assembly 10 and its corresponding parts will have thesame numerals but primed to indicate the analogy. Thus, when the outputof the first detector assembly 10 is overdriven by a premature explosionor is otherwise defective, the output of the second detector assembly10' appearing on cable 46', after being recorded by recorder 49, can beused to carry out the process of said U.S. Pat. No. 3,592,286.

While this invention has been described in connection with preferredembodiments thereof, it will be appreciated that modifications may bemade therein without departing from the scope of the claims attachedhereto:

What is claimed is:
 1. A method of marine seismic prospecting comprisingthe steps of:establishing a bubble containing a high-pressure expandingvolume of gas at a sufficient depth to prevent the gas from directlyventing to the atmosphere; the initial expansion of said gas bubblegenerating in the water a desired initial acoustic impulse forsubsequent reflection from the sub-bottom strata; said gas bubblesubsequent to said initial expansion collapsing and again expanding tocreate in the water at least a first bubble acoustic impulse which alsobecomes reflected from said sub-bottom strata; detecting the reflectedsignals from said initial impulse and said first impulse; and detectingthe unreflected acoustic impulses with at least one pressure detectorassembly, said detector assembly comprising:(a) a housing having aflexible, pressure-transmitting wall defining a chamber; (b) asubstantially incompressible dielectric liquid completely filling saidchamber, (c) a detector probe completely immersed in said liquid, saidprobe comprising: a metallic housing, a quartz crystal mounted on aninner wall portion of said housing, an amplifier in said housing, andcircuit means coupling said quartz crystal to the input circuit of saidamplifier; and recording at a remote location the output signals fromsaid amplifier, said output signals representing said unreflectedacoustic impulses.
 2. The method of claim 1 and from a remote locationfeeding current to said amplifier to energize same.